1. Field of the Invention
The present invention relates to a so-called CAD (Computer Aided Design) apparatus for designing a pattern of an electric circuit such as a pattern of a printed circuit board, a pattern of a hybrid IC (Integrated Circuit), etc., which is utilized to aid a designer to perform the designing operation of such patterns.
2. Description of the Related Arts
When the designer performs this kind of designing operation of the printed circuit board and the hybrid IC, the CAD apparatus by use of a computer, may be preferably utilized.
In the designing operations by use of such a CAD apparatus, the designer is supposed to directly obtain the design as for the selection of a material for a resistor (which will be referred as a "resistor material", hereinbelow) to be printed on a substrate etc. (which will be referred as "printed resistor", hereinbelow), as for the pattern shape of the printed resistor, and as for the pattern shape of an electrode for the printed resistor.
For example, the inventors of the present invention know one selecting method of such a resistor material, as following.
Namely, at first, the value of an aspect ratio As for each resistor material is calculated by the following expression (1), by use of a resistance R necessary for the printed resistor to be designed (which will be referred as a "necessary resistance" R) and a resistance Rs per unit area of various types of resistor materials to be used (which will be referred as a "sheet resistance" Rs). EQU As=R/Rs (1)
Then, the resistor material, which aspect ratio As calculated by the expression (1) is closest to "1", is selected as the most preferable resistor material.
On the other hand, the pattern shape of the printed resistor is obtained as following.
Here, as shown in FIG. 1, it is assumed that the shape of a printed resistor 17 is a basic type of a simple square plane shape, and that the printed resistor 17 is printed with each end thereof overlapped with each electrode 18 formed on the substrate.
At first, the value of a necessary minimum area Smn of the printed resistor 17 is calculated by the following expression (2), by use of an electric power Pp when the unit area of the resistor material selected by the above-mentioned manner is destroyed (which will be referred as a "permitted power" Pp, hereinbelow), an electric consumption power P of the printed resistor 17 to be designed (which will be referred as a "resistor consuming power" P), and a coefficient ft based on a trimming technique by the designer, which is basically obtained by his experiences. EQU Smn=ft.times.P/Pp (2)
Next, the values of an effective resistor width W and an effective resistor length L, are calculated by the following expressions (3) and (4), by use of the values of the aspect ratio As calculated by the expression (1) and the value of the necessary minimum area Smn calculated by the expression (2). EQU W=(Smn/As).sup.1/2 ( 3) EQU L=As.times.W (4)
Then, the pattern shape of the printed resistor 17 i.e. the value W of a printed resistor width W0 and the value "L+OVR.times.2" of a printed resistor length L0, and the pattern shape of the electrode 18 i.e. the value "OVR+M2" of an electrode width B, and the value "W+M1.times.2" of an electrode length A, are determined on the basis of the values of the pattern shape of the printed resistor 17 i.e. the effective resistor width W and the effective resistor length L calculated by use of the expressions (1) to (4).
In this manner, the selection of the resistor material, and the determination of the pattern shapes of the printed resistor 17 and the electrode 18 corresponding to the selected resistor material, can be performed.
However, the printed resistor 17, which pattern shape is set according to the values of the resistor shape data (the effective resistor width W and the effective resistor length L) calculated by use of the above-mentioned expressions (1) to (4), is actually subjected to the generation of some contact resistance at the overlapped portion 19 between the printed resistor 17 and the electrode 18, and also the generation of the printing blot at both ends of the printed resistor 17. Accordingly, there is a problem that an appropriate necessary resistance R cannot be in fact obtained according to the above mentioned design technique.
Thus, the designer may try to give some correction to the effective resistor width W and the effective resistor length L, which are calculated by use of the expressions (1) to (4), depending on his experience, so that an appropriate or tolerable pattern shapes of the printed resistor 17 and the electrode 18, can be determined as a final result of the designing operation.
In this way, according to the above explained CAD apparatus for designing the pattern of the electric circuit such as a printed substrate and a hybrid IC, the pattern shapes of the printed resistor and the electrode can be finally determined with the necessity of the experienced designer, so that the quality of the resultant pattern shapes depends on the skill of the designer in a great degree, and the objectively appropriate design can be merely obtained.